While I use "looks well" because of good being an adverb here, I mainly read "looks good" in the web.

I'm quite unsure about the ngrams chart, has the grammatically incorrect "looks good" simply superseded "looks well", as English became a world language? Or are there subtly (probably colloquial, slang) differences in meaning of both phrases?

Why do you think "looks good" is grammatically incorrect? "Good" is an adjective describing his appearance. Someone can be "happy" someone can be "good". If they look it, they either "look happy" or "look good".
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David SchwartzNov 9 '11 at 15:26

6 Answers
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In the sentences He looks well and He looks good both well and good are adjectives, assuming looks is a copular verb, that is, one that links other elements in the clause. The structure is not Subject-Verb-Adverbial, but Subject-Verb-Complement. The first sentence normally means that he seems to be in a good state of health. The second normally means that he has a pleasing appearance. The structure, but not necessarily the meaning, is the same as that found in He is well and He is good.

If looks is not a copular verb, He looks well would mean either that he has good vision, or that he surveys his surroundings with great care. However, both of those sentences are unlikely to be expressed in that way.

If you write, "Sally looks good in her new dress", "good" is an adjective. The construction is similar to if you wrote, "The dress looks blue".

If you write, "Sally looks well", people would normally understand you to mean "well" in the sense of healthy. Again, "well" is an adjective.

You might conceivably say "Sally looks well" using "well" as an adverb to modify how she looks. In that case, you are saying that Sally is doing a good job of looking. Such a sentence might be used to describe someone who is acting as a look-out, for example. It would be a similar construct to, "Sally cooks well" or "Sally drives well". But frankly, it's a rather unlikely thing for someone to say.

A common mistake in English is to use an adjective when you should have used an adverb, like "I speak English good" when you meant to say "I speak English well." But don't be confused by this and think you should always use an adverb after a verb! "Sally looks good" is correct if the intent is to say that Sally is pretty or otherwise to be favored. (Should I clarify that "... looks good" is often used other than in the sense of personal appearance? Like, "Which of the people who applied for this job should we hire?" "I think Mr Jones looks good." The meaning is that Mr Jones looks suitable for the job, not that he is handsome.)

Looks good can be taken literally, as in the opposite of evil. This is unlikely to be applied to "well" (where it means a water source). Well means healthy, fit, or as an abbreviation of well-proportioned, well-dressed, well-presented. You cannot substitute good there. Ergo "well" is the accurate grammar, "good" a mere colloquial form.